Liquid crystal display (LCD) panels, particularly color LCD panels, are used for flat screen televisions, projection television systems and camcorder view finders, with many more applications anticipated in the future.
The fabrication of an active matrix liquid crystal display typically involves several steps. In one step, the front glass panel is prepared. This involves deposition of a color filter element onto a suitable substrate, such as glass. Color filter deposition typically involves depositing a black matrix pattern and three primary (typically either red, green and blue in the case of transparent or substractive color filter) colors for color cell patterns within the spaces outlined by the black matrix. The printed lines which form the black matrix typically are about 15-40 microns wide and about 0.5 to 3 microns thick. The red, green, and blue color cells are typically on the order of about 70-100 microns in width by 200 to 300 microns in length. The color cells are typically printed in films less than about 10 microns thick, and preferably less than 5 microns thick, and must be evenly applied and accurately registered within the pattern formed by the black matrix. The front glass substrate is typically completed by depositing a planarizing layer, a transparent conducting layer, and a polyimide alignment layer over the color filter element. The transparent conducting layer is typically indium tin oxide (ITO), although other materials can also be utilized.
In a second step, a separate (rear) glass panel is used for the formation of thin film transistors or diodes, as well as metal interconnect lines. Each transistor acts as an on-off switch for an individual color pixel in the display panel. The third and final step is the assembly of the two panels, including injection of a liquid crystal material between the two panels to form the liquid crystal panel.
One critical step in the manufacture of the display is the preparation of the black matrix and color filter pattern. The sharpness of edge definition of the black matrix is extremely important. Unlike the colored ink cells, any variation in the black matrix edge, due to printing flow and so forth, is readily discernible when inspecting the final product. The color pixel edge, on the other hand, is typically hidden by the black matrix pattern. Consequently, to a certain extent the black matrix hides variability in the color pixel edge, while there is nothing to hide variability in the black matrix.
Consequently, black matrix patterns are typically prepared using photolithographic techniques, even where the remainder of the color filter pattern is produced using printing techniques. Photolithographic techniques involve a large number of production steps, and are much more complex than printing methods. In addition, photolithographic techniques are typically much more expensive than ink printing techniques.
Another critical step in color filter formation is the formation of the red, green and blue color dots (also referred to as color cells) of the color filter. Such color cells preferably should be deposited so that they are as smooth and uniform in thickness as possible. Previous attempts to print color filter patterns have resulted in color patterns having insufficient smoothness. This is largely because the ink depositing methods of the prior art resulted in ink cells which were rounded or triangular in cross section. Consequently, a planarizing layer is commonly applied over the color patterns to alleviate imperfections in coating smoothness or thickness uniformity due to the deposition process. The transparent planarizing layer also serves to protect against ion migration into the liquid crystal. The planarizing layer should be deposited to be as smooth and flat as possible.
To facilitate deposition of a flat planarizing layer, it is desirable that the color patterns be smooth, flat and substantially parallel to the undersurface of the glass substrate. Uniform thickness color patterns are desirable for obtaining optimum display contrast and color performance, because if the thickness of the pattern varies, the transmitted light intensity will vary.
One other problem with forming multicolored ink color filter patterns is preventing the different colored inks from mixing with one another. In the past, this problem has been solved by depositing the colors and drying and/or curing them one at a time.
U.S. Pat. Nos. 5,544,582 and 5,514,503 describe processes for making color filters wherein a multi-colored ink pattern is deposited onto a transfer layer which is carried by a collector roll. The composite multi-colored pattern/transfer layer is then transferred to a glass substrate to form a color filter.
Typically, the black matrix ink in such processes is transferred from an intaglio plate to the transfer layer. Prior to the transfer to the transfer layer, the black matrix intaglio plate is doctored with the intent of leaving ink only in the recesses. However, during this doctor blading step, the doctor blade can often leave a thin residue of black matrix ink between the intaglio recesses. If so, this residual black matrix ink ends up being transferred to the transfer layer into areas where the black matrix ink is not desired. A thin layer of black ink left on the top surface of the intaglio plate after doctoring is called haze. Transfer of this haze undesirably reduces transmission in the colored sub-pixel ink areas. Even when doctoring blade and operating parameters have been optimized for haze-free doctoring, the doctoring process is typically not a robust process for continuous printing of the black matrix for color filters. Particulates can get trapped behind the blade and cause clear streaks in the doctored black matrix. In addition, blades can become chipped, especially when doctoring intaglio plates constructed of hard materials. The resultant chips or grooves in the blade leave poorly doctored ink lines or black streaks in the regions intended to be clear. In order to achieve haze-free conditions, especially with fine particle inks, the blade pressures need to be high and this can result in chipping of the intaglio plate or degradation of the release coat on the intaglio plate.